Data networks used to transport data between distant end users can employ any of a number of technologies in transferring the data across intervening, interconnected systems. One well-known technology is frame relay technology which can be used to transport variable length data messages between the end users. In a frame relay network, a virtual link or virtual connection (VC) exists across the entire network between the communicating end users. This virtual connection provides the bi-directional transfer of data messages from an originating end user to a destination end user with the order preserved. This is done on the basis of a label attached to each data message called a data link connection identifier (DLCI). Each end associates the virtual connection with its own DLCI. The DLCI may be the same or different from the DLCI used at the other end.
Each DLCI is associated with a user-to-network interface (UNI). A UNI is a interface between a frame relay network and an end user. It includes a physical interface on the end user device, a physical interface on the frame relay network equipment, and the physical line between them.
Network nodes within the frame relay network communicate across network-to-network Interfaces or NNIs. The details of UNIs and NNIs are not important to an understanding of the present invention but may, if desired, be found in a number of documents available from known standards organizations. Representative documents include Frame Relay Forum FRF 1.1, Frame Relay Forum FRF 2.1, ITU X.76, ITU X.36, ITU Q.933 Annex A, and Frame Relay Forum FRF.10.
In known networks, data routing devices are typically interconnected using frame relay permanent virtual connections or PVCs. The descriptor "permanent" means that the resources for the connection remain committed for the life of the connection between the end users and are thus unavailable for use by others even when those resources are not actually needed. PVCs are an inefficient use of network resources and increase network operating costs for both network service providers and end users.
To provide more effective utilization of network resources, switched virtual connections (SVCs) can be implemented within and between frame relay networks. In the case of switched connections, network resources are committed for the duration of the connection. When there is no data to be transferred, the user can clear the virtual connection, releasing the network resources and making them available to other connections. Details of SVCs can be found in ITU X.36, X.76, Q.933; Frame Relay Forum FRF.4 and FRF.10. While SVCs are highly flexible, they are also costly to implement at a UNI and provide more functionality than is required by many existing network devices.
It is possible to provide a PVC service to the end users using SVC processing within and between frame relay networks. The resulting VC is called a switched permanent virtual connection (SPVC). The SPVC appears to the end users as a PVC, but is connected through the frame relay network(s) as an SVC. Although the SVC provides high resiliency and a reduction in network provisioning requirements, the service to the end user is still that of a PVC which means that network resources are committed to the VC even when there is no data to send. The end user has no means of notifying the frame relay network whether the resources of the SPVC are needed or not.